Image processing system for performing color image data calibration

ABSTRACT

An image reading apparatus measures, color by color, an average transmission-density signal based upon image information obtained from a color original through which light is transmitted, converts the measured average transmission-density signal of every color into a subject-lightness signal of every color, and effects a conversion into a reference color stimulus signal, which is indicated by the apex of a triangle substantially circumscribed by a spectrum locus of a predetermined chromaticity diagram, based upon the converted subject-lightness signal of every color.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus andmethod for reading and recording an image, for example, by using a colorexpression known as "c-RGB" (colorimetric RGB).

2. Description of the Related Art

A newly developed method of expressing colors involves obtaining theapex of a triangle substantially circumscribed on the spectrum locus ofa chromaticity diagram, and expressing color by a combination ofreference color stimulus data indicated by this apex. This method shallbe referred to as "c-RGB" hereinafter.

In a case where this method is applied to an apparatus for reading acolor film, however, the color of a subject in photography cannot bereflected correctly for reasons such as a disruption in the colorbalance on the film due to the influence of the light source, etc., atthe time of photography.

Further, in such a conventional color image processing apparatus,calibration is made in a signal form (RGB or YMCK) which is inherent toa color image reading unit or a color image recording unit. Thecalibration represents that a reading unit is corrected so that thesignal obtained when a color of known color characteristics is read inthe reading unit becomes an expected signal, and that a recording unitis corrected so that the color obtained when a predetermined signal isoutput to an image recording unit becomes an expected color.

However, the above color image processing apparatus has the problem thatsince calibration is made in a signal form inherent to the apparatus,when the reading unit and recording unit are replaced by another readingunit and recording unit, respectively, the color characteristics change.

In addition, if calibration is made for known NTSC (National TelevisionSystem Committee) RGB signals, NTSC correction must be made by raisingto 0.45 th power, and the operation is thus made troublesome. There isalso the problem that since the range of colors which can be expressedis limited, the color reproducible range of all reading units andrecording units cannot be expressed.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an imagereading apparatus in which the aforementioned drawbacks of the prior artcan be eliminated.

Another object of the present invention is to provide an image readingapparatus in which a c-RGB signal having excellent color balance at alltimes can be obtained from a measured value of the average density of afilm, irrespective of the type of film and the photographic conditions.

A further object of the present invention is to provide an image readingapparatus which performs highly precise color reproduction in conformitywith the characteristics of a color film reader.

Still another object of the present invention is to provide an imagereading apparatus comprising measuring means for measuring, color bycolor, a transmission-density signal based upon image informationobtained from a color original through which light is transmitted, firstconverting means for converting the transmission-density signal of everycolor, which has been measured by the measuring means, into asubject-lightness signal of every color, and second converting means foreffecting a conversion into a reference color stimulus signal, which isindicated by the apex of a triangle substantially circumscribed on aspectrum locus of a predetermined chromaticity diagram, based upon thesubject-lightness signal of every color resulting from the conversionperformed by the first converting means.

A further object of the present invention is to provide an imageprocessing apparatus and method which will not produce quantizationerror by using a color space formed so as to be circumscribed by aspectrum locus, and which permit calibration to be separately made foran image reading unit and an image recording unit.

A still further object of the present invention is to provide an imageprocessing apparatus and method which permit an increase in theprecision of calibration of each of the image reading unit and the imagerecording unit.

Another object of the present invention is to provide an imageprocessing method comprising the formation step of forming a color spaceso that a triangle formed by connecting the chromaticity points of theRGB primary colors on a xy chromaticity diagram is circumscribed by aspectrum locus of the chromaticity diagram, and the calibration step ofcalibrating an input/output system in accordance with the color spaceformed by the formation step.

A further object of the present invention is to provide an imageprocessing method comprising the formation step of forming a color spaceso that a triangle formed by connecting the chromaticity points of theRGB primary colors on a xy chromaticity diagram is circumscribed by aspectrum locus of the chromaticity diagram, the first calibration stepof calibrating a reading system in accordance with the color spaceformed in the formation step, and the second calibration step ofcalibrating a recording system using the reading system calibrated inthe first calibration step.

A still further object of the present invention is to provide an imageprocessing apparatus connected to an input/output unit, and comprisingformation means for forming a color space so that a triangle formed byconnecting the chromaticity points of the RGB primary colors on a xychromaticity diagram is circumscribed by a spectrum locus of thechromaticity diagram, and calibration means for calibrating theinput/output unit in accordance with the color space formed by theformation means.

A further object of the present invention is to provide an imageprocessing apparatus comprising an input/output unit, formation meansfor forming a color space so that a triangle formed by connecting thechromaticity points of the RGB primary colors is circumscribed by aspectrum locus of the chromaticity diagram, and calibration means forcalibrating the input/output unit in accordance with the color spaceformed by the formation means.

A further object of the present invention is to provide an imageprocessing apparatus connected to a reading unit and a recording unit,and comprising formation means for forming a color space so that atriangle formed by connecting the chromaticity points of the RGB primarycolors on a xy chromaticity diagram is circumscribed by a spectrum locusof the chromaticity diagram, first calibration means for calibrating thereading unit in accordance with the color space formed by the formationmeans, and second calibration means for calibrating the recording unitby using the reading unit calibrated by the first calibration means.

A further object of the present invention is to provide an imageprocessing apparatus comprising reading and recording units, formationmeans for forming a color space so that a triangle formed by connectingthe chromaticity points of the RGB primary colors on a xy chromaticitydiagram is circumscribed by a spectrum locus of the chromaticitydiagram, first calibration means for calibrating the reading unit inaccordance with the color space formed in the formation means, andsecond calibration means for calibrating the recording unit by using thereading unit calibrated by the first calibration means.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the construction of a color-filmreading apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a flowchart for describing the control operation performed bya CPU according to the first embodiment of the invention;

FIG. 3 is a block diagram showing the construction of the principalportion of a color-film reading apparatus according to a secondembodiment of the present invention;

FIG. 4 is a block diagram illustrating the construction of an imageprocessing apparatus according to a third embodiment of the invention;

FIG. 5 is a xy chromaticity diagram corresponding to color space signalsaccording to the third embodiment of the invention; and

FIG. 6 is a drawing showing the characteristics of LUT (Look-Up Table)according to the third embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described in detailwith reference to the accompanying drawings.

<First Embodiment>

The color-film reading apparatus of this embodiment utilizes the factthat the slope of a change in the density of a film with respect to theamount of exposure of the film to light is substantially constant fromone film to another, and the apparatus has a construction for obtaininga c-RGB signal having excellent color balance at all times from ameasured value of the average density of the film, irrespective of thetype of film and the photographic conditions.

The construction of the apparatus will now be described in detail.

FIG. 1 is a block diagram illustrating the construction of a color-filmreading apparatus according to a first embodiment of the presentinvention. Numeral 101 denotes a halogen lamp for transmissivelyilluminating film. The light from the lamp 101 is collimated andcondensed by a condenser lens 102 before passing through aheat-preventing filter 103. Numeral 104 denotes a color film originalthat is to be read. The film image of the color film original 104 isformed by an image-forming lens 105, and the film image formed isconverted into an electric signal by a photoelectric transducer 106. (Inthis embodiment, the photoelectric transducer 106 employs a three-lineCCD line sensor, hereinafter referred to simply as a "CCD", in which theelements are coated with films of the three colors R, G and B.) Anamplifier circuit 107 amplifies the RGB outputs of the CCD 106 in analogfashion, and the resulting analog outputs are sampled and held by asample-and-hold (hereinafter referred to as "S/H") circuit 108. Theanalog outputs of the S/H circuit 108 are converted into digital signalsby an analog/digital (hereinafter referred to as "A/D") converter 109. Ashading correction circuit 110 corrects for irregularities in thesensitivity and illumination of the CCD 106. A shading RAM 111 holdscorrection data for the shading correction. One line each of thecorrection data is held with regard to each of the colors R, G, B. Alogarithmic converting circuit 112 for logarithmically converting theRGB signals has a look-up table (hereinafter referred to as a "LUT") inwhich data is capable of being written from a CPU 116. Aconstant-voltage power supply (hereinafter referred to as a "CVR") 113and a D/A converter 114 allow the ignition voltage of the light source101 to be controlled by the CPU 116. A D/A converter 115 is forcontrolling the amplification factor of the amplifier 107 independentlyfor each of the colors R, G, B. The CPU (central processing unit) 116 isconnected to the logarithmic converting circuit 112 and to the D/Aconverters 114, 115 and transmits data to each of these circuits. Amatrix converting circuit 117 converts the logarithmically convertedsignal from the converting circuit 112 into a c-RGB signal. Numeral 118denotes an interface (hereinafter referred to as an "I/F") fordelivering output signals. The arrangement is such that the color filmoriginal 104 is scanned in the direction of the arrow by a scanningmechanism (not shown) so that the entirety of the image is read.

By virtue of the arrangement described above, the average transmissiondensity of the color film original 104 is measured, after which the CPU116 executes prescribed processing. This is processing through which theR, G, B signals are made into ones proportional to the luminance of thesubject, namely the reflectance of the subject. Subject reflectance rand subject density d are related by the equation d=-log r. If we letr_(min) represent the reflectance of the darkest portion that is to bereproduced, the relation d_(max) =-log r_(min). Since the log ofexposure E in an ordinary exposure region and the log of filmtransmittance T are linearly related, an equation of the minimumquantization error is expressed as follows:

    d=d.sub.max +(1/γ.sub.i)logT.sub.i                   (1)

where i represents R, G, B, and γ (gamma) represents the slope of thestraight line. When Equation (1) is used, γ can be found by thefollowing equation:

    r=r.sub.min (T.sub.i).sup.-1/γi                      (2)

Here a LUT for realizing Equation (2) is written in the logarithmicconverting circuit 112 in advance. Subject luminance information isreproduced using the LUT, after which a matrix conversion to the c-RGBsignal is performed by the matrix converting circuit 117.

The operation of this embodiment will now be described.

FIG. 2 is a flowchart for describing the control operation performed bythe CPU 116 according to the first embodiment. In the following, onlythe main processing is described. Each processing operation is performedby a corresponding circuit, and the CPU 116 controls the flow of theoverall processing.

First, the average density of the color film is read by the CCD 106 atstep S1, and then the average density is converted into the subjectluminance signal by the logarithmic converting circuit 112 at step S2.Next, at step S3, the subject luminance signal is converted intoreference color stimulus data, which is indicated by the apex of atriangle substantially circumscribed by a spectrum locus of apredetermined chromaticity diagram, by the matrix processing of thematrix converting circuit 117. The stimulus data obtained as a result ofthis processing is outputted via the I/F 118 at step S4.

Thus, in accordance with this embodiment as described above,color-separation signals having an excellent color balance at all timesare obtained irrespective of the type of film and the degree oforange-masking. Moreover, the embodiment makes possible colorreproduction in which the quantization error of digital processing isminimized.

<Second Embodiment>

In this embodiment, the matrix converting circuit for effecting theconversion to the c-RGB signal can be constructed as a LUT.

FIG. 3 illustrates the construction of the principal portion of thecolor film reading apparatus according to a second embodiment of theinvention. Numeral 217 denotes a matrix converting circuit according tothis embodiment. The converting circuit 217 has a LUT 218 in which datais capable of being written from a CPU, not shown.

The three-color separation system of the color film reading apparatusand the sensitivity characteristics of the film often are displaced fromthe color-matching function. In such case, it is impossible to perfectlyperform the conversion into the c-RGB signal by means of a matrixconversion. For this reason, the characteristics of the color filmreading apparatus are put into the form of a LUT in advance, therebymaking possible highly precise color reproduction.

<Third Embodiment>

FIG. 4 is a block diagram illustrating the construction of an imageprocessing apparatus according to a third embodiment of the invention.As shown in FIG. 4, the image processing apparatus mainly comprises animage reading unit 304, an image processing unit 310 and an imagerecording unit 305. The whole system is controlled by a CPU 307.Reference numeral 303 denotes an original, and reference numeral 306denotes a copy. The image processing unit 310 has a 3×3 matrixcalculator 301 and a LOG 3×3 conversion and masking processor 302.Reference numeral 308 denotes a ROM for storing a program the CPU 7executes, and reference numeral 309 denotes a RAM used as a working areafor each of the programs.

FIG. 5 is a xy chromaticity diagram corresponding to a color spacesignal according to the third embodiment, and FIG. 6 is a drawingshowing the characteristics of a LUT (Look-Up Table) according to thethird embodiment.

In this embodiment, the color space signal shown in the xy chromaticitydiagram of FIG. 5 is generated between the 3×3 matrix calculator 301 andthe LOG conversion and masking processor 302, both of which will bedescribed below. The color space signal is defined as the c-RGB signal.

In the xy chromaticity diagram shown in FIG. 5, the positions P1, P2 andP3 at the apexes of a triangle are considered as the reference colorstimulus positions. This embodiment employs a color expression method inwhich the stimulus positions on the xy coordinates are as follows:

R: (0.7347, 0.2653)

G: (-0.0860, 1.0860)

B: (0.0957, -0.0314)

The triangle formed by connecting these three positions substantiallycontacts the spectrum locus of the xy chromaticity diagram atwavelengths of about 505 nm and 525 nm, and substantially overlap thelocus of pure purple formed by connecting 380 nm and 780 nm.

Namely, when the reference color stimulus positions are at the apexes ofthe triangle circumscribed by the spectrum locus of the chromaticitydiagram so as to include the spectrum diagram, a color specificationscheme using signals in which chromaticity values outside the spectrumlocus, i.e., nonexistent colors, are eliminated as much as possible, canbe obtained. The color matching functions are determined by thereference color stimuli, and the c-RGB signal value of a color eachhaving the known color characteristics is determined by calculationusing the matching function. This permits the calibration below.

In the apparatus shown in FIG. 4, color patches of plural colors eachhaving the known color characteristics (spectral sensitivity,chromaticity) are read by the image reading unit 304. The 3×3 matrixcoefficients are determined so that the signal value (A') obtained byprocessing the RGB signal in the 3×3 matrix processor 301 substantiallyagrees with the c-RGB signal (A) determined by the calculation using thematching functions. The calculation for determining the coefficients ismade by the CPU 307. The signals A' and A are stored in the RAM 309.Alternatively, the signal A may be previously stored in the ROM 309. Thecoefficients are determined by a least square method or the like. Thecalculation using the color matching functions is described below. It isassumed that the spectral reflectance of the color patch is P(λ), thematching functions are r(λ), g(λ) and b(λ), the spectral distribution ofa CIE (International Commission on Illumination) standard light sourceD65 is S(λ), and normalizing white color is W(λ), and a reflectance ofabout 70% to 90% is used. Assuming that the c-RGB values are R, G and B,the following equations are established:

    R=255*ΣP(λ)*r(λ)*S(λ)*/ΣW(λ)*r(.lambda.)*S(λ)

    G=255*ΣP(λ)*g(λ)*S(λ)*/ΣW(λ)*g(.lambda.)*S(λ)

    G=255*ΣP(λ)*b(λ)*S(λ)*/ΣW(λ)*b(.lambda.)*S(λ)                                          (3)

In the above equations (3), the coefficient 255 is applied to a 8 bitsignal having gradation of 0 to 255, and Σ represents the sum ofproducts of the wavelengths from 380 nm to 780 nm. The coefficient 255is not limited to 8 bits, it may be applied to, for example, 16 bitsother than 8 bits, and the range of Σ can also be variously changed.

In this way, the image reading unit 304 is calibrated.

The calibration of the image recording unit 305 is described below.

The combination (A) of the c-RGB signals of plural colors which aredetermined by the above calculation is input to the image recording unit305, and a hard copy is output from the image recording unit 305. Thespectral reflectance of the color of the hard copy is measured and isthen substituted for P(λ) of the above equation (3) to determine each ofthe R, G and B values. If the combination of the thus-determined R, G,and B signal values is represented by H, the CPU 307 determinesparameters of the LOG conversion and masking processor 302 so that thecombination H is close to the combination A. The signal combinations Hand A are stored in the ROM 309. The image recording unit 305 may becalibrated using any desired signal combination other than thecombination A.

Denoting input as I, and output as θ, the LOG conversion is expressed bythe following equation:

    θ=(-255/Δ/D)log I/255                          (4)

The RGB signals are converted into CMY signals by the LOG conversion.The masking represents that, for example, a 4×4 matrix operation is madeusing K and CMY values in the equation K=min(Y, M, C).

As described above, this embodiment uses as a color space for signalsused for calibration the color space formed so that a triangle formed byconnecting the chromaticity points of the primary colors on the xychromatic diagram is circumscribed by the spectral locus, and theembodiment thus has the effect of enabling calibration usefully usingcolor data with producing less color error.

A calibrated image reading unit 4 and a 3×3 matrix processor 1 may beemployed for measuring colors.

Alternatively, when the image recording unit 5 is calibrated, a methodin which 2 to 255 continuous gradation patterns of each of the colorsYMCK are outputted, the output patterns are read by the image readingunit 4 and then the patterns are converted into c-RGB signals may beused for calibrating the gradation of each of the colors YMCK. In thiscase, a LUT (Look-Up Table) of each color having the characteristicsshown in FIG. 6 may be used.

The order of the 3×3 matrix operation can be increased to 3×3 or more,and a LUT may be prepared for converting the output from the imagereading unit into c-RGB signals. The LUT may also be used for convertingthe c-RGB signals into YMCK signals in the LOG conversion and maskingprocessor.

As described above, the third embodiment uses as a color space ofsignals used for calibration the color space formed so that a triangleformed by connecting the chromaticity points of the primary colors onthe xy chromaticity diagram is circumscribed by the spectrum locus, andthus has the effect of enabling calibration with high precision.

<Modification of Third Embodiment>

A modification of the third embodiment is described below. Since thecomponents of an image processing apparatus according to this modifiedembodiment are the same as those of the image processing apparatusaccording to the above embodiment, the components are not describedbelow and shown in the drawings.

In this modified embodiment, a 3×3 matrix is previously prepared in theimage reading unit, and the output signals are converted into c-RGBsignals by calibration. The LOG conversion and masking processor isincorporated into the image recording unit so that LOG conversion andmasking are performed, and the LUT of gradation is determined by usingthe calibrated image reading unit. The calculation is performed by theCPU of the image reading unit or the image recording unit.

In addition, the quantization error of the signal values can bedecreased by setting the white level to 75 to 90% which is thereflectance of usual white paper.

In the present invention, the masking may include processing such as UCRand employ black signals, or the masking and UCR may be separatelyperformed. In addition, the order of making may be second or more.

The present invention can be applied to a system constituted by aplurality of devices or to an apparatus comprising a single device.Furthermore, it goes without saying that the invention is applicablealso to a case where the object of the invention is attained bysupplying a program to a system or apparatus.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

What is claimed is:
 1. A color image processing apparatuscomprising:obtaining means for obtaining color signal data correspondingto a predetermined color by using an input system, the color signal databeing expressed by a combination of three stimulus data corresponding tovertices of a triangle which substantially contacts a spectrum locus ofan xy chromaticity diagram at wavelengths of about 505 nm and 525 nm,and substantially overlaps the locus of a purple boundary formed byconnecting 380 nm and 780 nm; and calibration means for calibrating theinput system so that the color signal data is substantially equivalentto expected color signal data of the predetermined data.
 2. A colorimage processing apparatus according to claim 1, wherein said inputsystem has a reading unit.
 3. A color image processing apparatusaccording to claim 1, wherein said obtaining means obtains the colorsignal data using a conversion matrix, and said calibrating meanscalibrates elements of the conversion matrix.
 4. A color imageprocessing method comprising the steps of:obtaining color signal datacorresponding to a predetermined color by using an input system, thecolor signal data being expressed by a combination of three stimulusdata corresponding to vertices of a triangle which substantiallycontacts a spectrum locus of an xy chromaticity diagram at wavelengthsof about 505 nm and 525 nm, and substantially overlaps the locus of apurple boundary formed by connecting 380 nm and 780 nm; and calibratingthe input system so that the color signal data is substantiallyequivalent to expected color signal data of the predetermined color. 5.A color image processing method according to claim 4, wherein said inputsystem is a reading system.
 6. A color image processing method accordingto claim 4, wherein said obtaining step obtains the color signal datausing a conversion matrix, and said calibrating step calibrates elementsof the conversion matrix.
 7. A color image processing method accordingto claim 4, wherein the three stimulus data are R: (0.7347, 0.2653), G:(-0.0860, 1.0860) and B: (0.0957, -0.0314) on the xy chromaticitydiagram.
 8. A color image processing apparatus comprising:obtainingmeans for obtaining color signal data corresponding to a predeterminedcolor, the color signal data being expressed by a combination of threestimulus data corresponding to vertices of a triangle whichsubstantially contacts a spectrum locus of an xy chromaticity diagram atwavelengths of about 505 nm and 525 nm, and substantially overlaps thelocus of a purple boundary formed by connecting 380 nm and 780 nm;output means for outputting a color image in accordance with the colorsignal data using an output system; and calibration means forcalibrating the output system so that the output color image issubstantially equivalent to the predetermined color.
 9. A color imageprocessing apparatus according to claim 8, wherein said output systemhas a recording unit.
 10. A color image processing apparatus accordingto claim 8, wherein said three stimulus data are R: (0.7347, 0.2653), G:(-0.0860, 1.0860) and B: (0.0957, -0.0314) on the xy chromaticitydiagram.
 11. A color image processing method comprising the stepsof:obtaining color signal data corresponding to a predetermined color,the color signal data being expressed by a combination of three stimulusdata corresponding to vertices of a triangle which substantiallycontacts a spectrum locus of an xy chromaticity diagram at wavelengthsof about 505 nm and 525 nm, and substantially overlaps the locus of apurple boundary formed by connecting 380 nm and 780 nm; outputting acolor image in accordance with the color signal data using an outputsystem; and calibrating the output system so that the outputted colorimage is substantially equivalent to the predetermined color.
 12. Acolor image processing method according to claim 11, wherein the outputsystem is a recording system.
 13. An image processing apparatuscomprising:generating means for generating correction data forconverting first color image data represented in a first color space,which depends on an image reading unit, to second color image datarepresented in a second color space, which is formed as a triangle on axy chromaticity diagram, said triangle substantially contacting aspectrum locus of the xy chromaticity diagram at wavelengths of about505 nm and 525 nm, and substantially overlapping the locus of a purpleboundary formed by connecting 380 nm and 780 nm; input means forinputting first color image data in the first color space from the imagereading unit; conversion means for converting the first color image datato second color image data in the second color space based on thecorrection data generated by said generating means.
 14. An imageprocessing apparatus according to claim 13, wherein said generatingmeans generates correction data based on third color image data andfourth color image data, wherein the third color image data isrepresented in the first color space and obtained by reading an originalimage using the image reading unit, and the fourth color image datarepresents the original image in the second color space.
 15. An imageprocessing apparatus according to claim 13 wherein said converting meansperforms matrix operation.
 16. An image processing method comprising thesteps of:generating correction data for converting first color imagedata represented in a first color space, which depends on an imagereading unit, to second color image data represented in a second colorspace, which is formed as a triangle on a xy chromaticity diagram, saidtriangle substantially contacting a spectrum locus corresponding to areal color region of the xy chromaticity diagram at wavelengths of about505 nm and 525 nm, and substantially overlapping the locus of a purpleboundary formed by connecting 380 nm and 780 nm; inputting first colorimage data in the first color space from the image reading unit;converting the first color image data to second color image data in thesecond color space based on the correction data generated in saidgenerating step.
 17. An image processing method according to claim 16,wherein the correction data is generated, in said generating step, basedon third color image data and fourth color image data, wherein the thirdcolor image data is represented in the first color space and obtained byreading an original image using the image reading unit, and the fourthcolor image data represents the original image in the second colorspace.
 18. An image processing method according to claim 16, whereinmatrix operation is performed in said converting step.